1. Field of the Invention
The present invention relates to systems and methods for the separation and washing of clay, dirt, and other impurities from gravel, and particularly to a log washer which has a sectioned shaft with groups of offset paddles, wherein paddles within each group are further offset from each other.
2. Description of the Related Art
With the increase in the number of large scale construction projects, as well as landscaping projects that are currently in development, there has been a corresponding increase in the demand for gravel. Gravel is a widely used material in the modern construction industry. As a component for concrete, roadways, and other materials, gravel is needed for the expansion of future constructive endeavors. Gravel is often obtained conventionally by using strip mining techniques in quarries. In this operation, large amounts of earth and rock from the barren Earth are removed in the long strips. The resulting gravel from this material, however, often has much unwanted debris, such as sticks, roots, and clay. In order to prepare gravel for the commercial market, it is necessary to wash and remove these contaminant materials from the gravel.
Log washers derive their name from the early practice of putting short lengths of wood logs inside a rotating drum with sand and gravel to aide in the scrubbing motion of various aggregate materials. Despite the name's suggestive nature, log washers do not wash timber. Instead, today's log washers are a commonly used mechanical device in the quarry industry, being used to break down and separate clay, dirt, agglomerated rock, organic waste material, and other impurities from aggregates such as gravel and sand.
Typical log washers have a tank or trough mounted on an incline relative to the horizontal at an angle of between about five to ten degrees, with eight degrees being typical. A pair of parallel shafts, horizontally spaced from each other, run along the lengthwise direction of the tank rotate freely. A motor is connected to one end of the shafts by an appropriate gearing to rotate each of the shafts in opposite directions. Each shaft has a plurality of paddles mounted thereon, usually mounted to the shaft at an angle. The paddles on adjacent shafts are staggered such that the intermeshing nature of the counter-rotating shafts subject the aggregate material to a persistent abrading and grinding action which separates the gravel from the undesired material. Feed material is fed to the lower end of the log washer by the use of an appropriate hopper or conveyor belt, while the paddles then mesh the aggregate material and slowly move the gravel towards the upper end of the tank. The clean gravel is then discharged out of the bottom of the upper end of the tank while the undesired material are absorbed by the water and carrier over a weir over either side of the box.
A problem with conventional log washers, however, is the placement of the paddles along the shaft. For example, the paddles are often mounted in straight rows along the length of the shaft. Upon each complete rotation of the shafts, and therefore the paddles, the entire load in the tank is tossed laterally against the entire sidewalls of the tank. This lateral shifting of the tank creates significant vibrations which reverberate along the entire length of the tank. These reverberations cause the shafts and tank's support structures to experience much stress and strain due to the persistent rocking back and forth motion caused by the counter-rotating shafts.
The greater the forces involved, the studier the shaft and shaft tank support structure must be to withstand the persistent reverberations. In many instances, log washers can be multiple stories tall. As such, the support structure needed to absorb the reverberations has to be very substantial to prevent the log washing structure from toppling over. This greatly increases the cost of log washers and, in particular, large log washers used in large scale gravel quarrying facilities.
Log washers have been developed in the past that address the vibration issue by arranging the paddles helically along the length of the shaft. This results in substantially reduced vibration but reduces the disruptive force being exerted on the slurry material as only a single pair of paddles is creating a proximate interface in the slurry at one time. As such, log washers with helically arranged paddles are less effective at dislodging debris from gravel than the traditional log washers having parallel rows of paddles.
Several systems have been proposed which reduce the amount of reverberations felt by the log washer tank. For example, U.S. Pat. No. 6,752,274 B2, to Mirras, entitled, “Log Washer with Staggered Paddles,” discloses a log washer where the paddles are positioned in groups and the groups are staggered along the shaft to reduce the shifting load stress and strain in the log washer support structure. By staggering the paddles in such a fashion, only a portion of the load in the tank, as opposed to the entire load in the tank, is thrown against a section of the sidewalls of the tank at any one time upon the rotation of the shafts. By reducing the total load thrown against the sidewalls of the tank on each shaft's rotation, it reduces the intermittent shock loading imposed on the shafts and log washer's support structure. Distributing the stresses generated by the shifting load over the length of the tank and over time further reduces the mechanical vibrations felt by the shafts and tank support structure.
A further problem with conventional log washers is the relatively low throughput of the gravel or sand. While the log washer disclosed in the Mirras patent reduces the amount of vibration over log washers having parallel rows of paddles, Mirras does not provide an increase in the throughput of the material nor does Mirras increase the turbulence of the water flow to dislodge the gravel.
From the foregoing it should be appreciated that there is a need for a log washer that is less susceptible to vibration but also has an increased throughput of material. To this end, there is a need for a log washer that reduces vibration but have paddles that cooperate with each other to enhance the flow of material along the length of the trough.